so and such

Why do these two little function words get a short guide to themselves?
In fact, they are analysed elsewhere, in different guides on this site, and the reason for that is to do with something called gradience.
Both these words, especially so, slide between word classes and perform a number of grammatical functions. There are links at the end taking you to the guides to those functions (and to gradience in general concerning how other function words are affected). What happens here is that we look at the two words independently to see what they can do.
The reason for that is that the words cause considerable confusion for learners of English and teachers need to be alert to what the words can do and how the structures we use with them are constrained.
Briefly, the words may act in the following grammatical functions as:

modifiers
e.g.:
She's so impatient
That's such a lovely suit
pro-forms
e.g.:
That's a new machine and as such it should work properly
He asked me to explain the ideas and I did so
conjunctions
e.g.:
    The rain was pelting down so that I couldn't see through the window
    He left early so I didn't have time to ask him
    I painted the door carefully three times such that it really looked the part
conjuncts (not such)
e.g.:
    So, what can I do for you?
    So, that's your car is it?
    So, what do we think?

We will discover, throughout what follows that the essential difference between these two word is that such is generally a noun modifier or a nominal expression itself and so acts more usually adverbially or even verbally.

As we go along, we'll look a little at how other languages deal with the concepts and that will help us to be alert to inter-language errors.

Modifiers

Both so and such are used as emphasising or amplifying pre-modifiers or as expressions of degree but they work differently. They both carry two related meanings:

The meaning of very as in, e.g.:
That is so beautiful
That is such a nice view
The meaning of to that degree or extent as in, e.g.:
The traffic was so bad that it took an hour to get there
It was such a nice hotel that we decided to stay another night

This difference in meaning should not be underestimated. It is often ignored in course materials and it can lead to errors in production and especially in reception if learners are told (or allowed to assume) that both words always mean very or something like it.
Incidentally, the so ... that structure in the last example above is not of the structure acting as a conjunction, whatever you may read on the web or find, alas, in coursebooks. That is explained below. Here, it is an example of so modifying the adjective bad and the that-clause is simply the complement.

As modifiers in sense a. above, when the words work as simple amplifiers, the situation is quite straightforward.

so pre-modifies adjectives and adverbs as in:
    This is so beautiful
    He drives so quickly
and in this meaning it is not usually followed by a that-clause because it only means something like very.
When we follow it with a that-clause, the meaning slides towards sense b. and becomes akin to to that degree. For example:
    This is so badly made
just means:
    This is very badly made
but
    This is so badly made that it will fall apart
means
    This is badly made to the extent that it will fall apart.
such pre-modifies noun phrases only as in:
    It was such a good performance
which just means
    It was a very good performance
and in
    That was such a nice meal
which just means
    It was a very nice meal
and, again, in this meaning no that-clause is needed because the word is a simple amplifier.
When we follow it with a that-clause, the meaning again shifts to something akin to to that degree. For example:
    It was such a good performance that I'm going again tomorrow
    That was such a nice meal that I'll try cooking it at home

The rule is:

As modifiers, so and such without a that-clause mean very but the same words with a that-clause mean to the extent or degree that.

In other languages, the meaning is usually to the extent that rather than a simple amplifier but there are exceptions (Japanese, for one).

The grammar as modifiers

so has more than one use as an adverbial modifier
- so pre-modifies an adjective or adverb phrase to add emphasis as in, e.g.:
  The meal was so badly cooked that he refused to pay
  The man was so often dishonest that no-one believed anything he said
  so cannot post-modify phrases.
- When so is used in the initial position as an emphasiser, it requires the inversion of subject and verb:
  So stupid was the idea that nobody took it seriously
such has a number of uses in modification:
- such pre-modifies a noun phrase to add emphasis and acts, therefore, as a pre-determiner as in, e.g.:
  It was such a bad meal that he refused to pay
  The man was such a liar that no-one believed anything he said
- When such is used in the initial position as an emphasiser, it requires the inversion of subject and verb:
  Such was his dishonesty that no-one believed anything he said
- such can also act as a determiner in its own right as in, e.g.:
  I don't trust such people
  in which there is no sense of added emphasis and such means something like of that sort.
- such can also act as a post-modifier of a noun phrase as in, for example:
  Liars such as he can never be believed
  In this case, the use of as with such is compulsory and there is, again, no emphatic effect and the meaning is like him.
  (In fact, the pronoun is often put colloquially, some say erroneously, in the object case after such as in:
  Women such as her are very valuable to the company
  by analogy with the object-case pronoun after the preposition like.)

Many languages do not distinguish between the two ideas with the use of a different adverb or determiner.
For example, if we translate these two words:

German uses so in both cases
Spanish uses tan in both cases
French uses si in both cases
Czech uses tak in both cases
Greek uses tóso in both cases.
Chinese languages, by contrast, will use the same way to express the ideas (with either zhème or nàme) but distinguish between things distant, in time or place, and things nearby or recent.
Japanese expresses the concept but generally conflates the use with very.

There are differences, too, in the functions that the two words can perform as pre-modifiers.

such can be a pre-determiner, as we saw, coming before another determiner so we can have, for example,
It was such a beautiful day that we went for a walk
so, on the other hand cannot do this because its nature is adverbial in this sense. The article determiner sticks with the noun, so modifies the adjective or adverb and does not function as a pre-determiner. For example:
It was so beautiful a day that we went for a walk
The sun was shining so strongly that I had to get into some shade
However, so can be a pre-determiner of a quantifier determiner and we allow:
    There was so much water in the garden that it was impossible to dig
    We have so few friends now that we've moved to America
etc.
Other quantifiers routinely modified with so include little and many but neither so (nor much) can modify the same words when they are preceded by the indefinite article so,
    *so a few
    *so a little
etc. are not available.
Informally, so can also pre-modify the otherwise barely modifiable adverb very. We hear, but seldom read, therefore:
It was so very kind of you to invite me
This modification often occurs in negative clauses when, perversely, it acts to tone down rather than emphasise the adverb as in, e.g.:
    It wasn't so very difficult to do
    It's not so very different
etc.
Corpus research reveals that expressions such as:
    so very grateful
    so very happy
    so very helpful
etc. are very frequent in informal speech and writing.

Finally, in terms of modification, the co-text often includes a that-clause as the consequence of the heightened nature of the noun, adjective or adverb phrase as we see above.
This, however, only happens when the words mean to that extent or degree and not when the words imply simply very.

Recently, by which is meant more or less in the last ten years, the word so has taken on the role of an adverb meaning something like greatly. We often hear (but seldom see), therefore:
I so like your dress
I so dislike waiting in a queue
etc.
Whether this extension of the word's natural meaning will become embedded in the language is anyone's guess but it is a perfectly logical one.

Pro-forms

Pro-forms, to which there is a guide linked below, act to replace an item to avoid stylistically unacceptable repetition or simply to make the language more concise and more easily produced. Pronouns are a common form of pro-form but by no means the only one.

Both so and such can act as pro-forms but the grammatical functions they perform are different and, as we have come to expect, such performs a nominal function and so performs a verbal or clausal function.

such can act as a pro-form for a noun phrase, usually allied with the preposition as. For example:
The shopkeeper was a man of eccentric tastes and as such he had a huge range of stuff for sale
in which such stands for a man of eccentric tastes
You are the manager and as such it's up to you to decide.
in which such stands for the manager
so can act as a pro-form for a verb phrase and its object if any
    He told me to get two bottles and I did so
in which so stands for get two bottles
    Is Peter coming to the party? Mary said so.
in which so is a pro-form for Peter is coming to the party
    Is it worth the money? I think so.
in which so is a pro-form for it is worth the money
    I'm not a child sand you shouldn't treat me so
in which so is a pro-form for as if I were a child
in all these cases, so is a pro-form standing for an entire clause.

Other languages do not use the same words for the functions of pro-form, pre-determiner, determiner or adverb. For example, most languages would usually prefer a pronoun translatable as it or that to stand for a clause as in:
He asked me to get the money and I did that / it.
which is possible in English but unnatural and we would usually prefer:
He asked me to get the money and I did so.

Conjunctions

Both words can act as causal or resultative conjunctions but so is much more frequent and occurs with and without that.

We can have, therefore:
I nailed it down firmly so that it couldn't come loose again
in which so that expresses the purpose and may be replaced by so on its own so we could have:
I nailed it down firmly so it couldn't come loose again
with exactly the same meaning.
In these cases, so that and so are acting as subordinating conjunctions of purpose.

The conjunction so that (never separated) is more rarely used as a coordinating conjunction as in, e.g.:
The car broke down so that I couldn't get to the wedding.
and here it refers to a result, not a purpose.
Compare, for example:
The machine was broken so that we couldn't finish the job
which has two possible interpretations:

Someone deliberately broke the machine in order that the job could not be finished
The result of the machine being broken was that the job could not be finished

In interpretation 1., the use of so that indicates purpose and is subordinating.
In interpretation 2., the use of so that indicates result and is coordinating.

The conjunction so that signalling result is a coordinator because it cannot be preceded by another coordinator. We cannot allow, e.g.:
*There was snow on the ground and so that I could see which way the dog went
However, the conjunction so is a subordinator and can be preceded by a coordinator. We allow, therefore:
There was snow on the ground and so I could see which way the dog went.

The conjunctional use of so that is different structurally and semantically from the use of so as a modifier of an adjective or adverb followed by a complement that-clause.
As a modifier, the words is separated from the that-clause by the adjective or adverb as in, e.g.:
    He played so beautifully that the audience almost cried
    They were so cheap that I bought three
and the meaning is to the extent that.
As a conjunction the words so and that come together forming a complex conjunction as in, e.g.:
    There had been heavy rain for days so that the ground was waterlogged
in which the conjunction means with the result that.
This matters in the classroom because the meanings are different and the word ordering is important. Not to distinguish encourages errors such as:
    *There had been so heavy rain for days that the ground was waterlogged
or
    *They were cheap so that I bought three

Such can also be a coordinating conjunction of sorts as in, e.g.:
I nailed it down firmly such that it couldn't come loose
In this, rather rare, use of such, an alternative analysis is that such is a pro-form for an ellipted noun phrase and the idea is often more easily expressed as
I nailed it down firmly in such a way that it couldn't come loose.
In this analysis, such is a pro-form for in a way.

The conjunctions so and so that are also discussed in separate guides to coordination and subordination linked in the list of related guides at the end.

Conjuncts

A conjunct acts to link independent clauses anaphorically and there is a guide to them linked in the list at the end. Briefly, a conjunct exists outside the clauses it connects and can be omitted to leave well-formed language. For example, we can omit so in:
    I was hungry and so I had an early lunch
to leave
    I was hungry and had an early lunch
because the word is a conjunct and exists outside both clauses, but we cannot omit the word from:
    I was hungry so I had an early lunch
because it is a subordinating conjunction integral to the second clause and it leaves the ungrammatical:
    *I was hungry I had an early lunch

Only so can act as a conjunct rather than a conjunction. The word such never performs this grammatical function.
In this form, so has these meanings:

To express a result
I overslept, couldn't find a clean shirt, missed my train and had to walk to work. So, I was pretty late.
in which so acts to connect the second sentence to the first anaphorically and means consequently.
To express a logical conclusion
So, she thinks she can do it better, does she?
So, this is where he lives
in which so implies that the speaker is reaching a conclusion from what has been said and means something akin to in that case I conclude.
To sum up
So, where have we got to?
in which the speaker is about to sum up or recapitulate the arguments or is inviting someone else to do so and it means something akin to now that we have got all that behind us.

There is more on conjuncts in the guide linked in the list of related guides at the end.

Teaching the words

Despite what it may say in your coursebook, do not be tempted to teach these words together or jumble the meanings in a single presentation. The cross-over between them confuses more than it enlightens and, as we saw, the temptation to translate is not one with fruitful outcomes.
A source of materials-induced errors is the habit of some coursebook writers to conflate the meanings of so and such as simple amplifiers with their meanings of to the extent or degree that. So, that way, as such, lies madness.

However, quite frequently, problems with use and meaning arise in the classroom incidentally to the lesson's aims and the issues do need dealing with.
This can't be done, naturally, by any teacher who hasn't arrived at a satisfactory understanding of how the words work and what their communicative and grammatical functions are in their various guises.

Related guides
gradience	to see what other function words can be affected by difficulties assigning word class
conjunction	for a general guide with links to specific areas
conjuncts	for a guide to a special form of adverbial
coordination and subordination	for a general guide with links to specific areas
determiners	for more on this word class
pre- and post-determiners	for more on a closed group of words that can occur before a determiner
adverbials	for more on conjuncts, adjuncts, disjuncts and so on
pro-forms	for a guide to this area of substitution